1. Field of the Invention
The present invention relates to a semiconductor device including a vertical insulated gate bipolar transistor (vertical IGBT) and a vertical free-wheeling diode (vertical FWD) formed in one semiconductor substrate.
2. Description of the Related Art
In a conventional reverse-conducting IGBT device (RC-IGBT device), a vertical FWD and a vertical IGBT are formed in one semiconductor substrate as described, for example, in US 2005/0017290 A (corresponding to JP-A-2005-57235) and US 2008/0048295 A (corresponding to JP-A-2008-53648).
In the RC-IGBT device, an anode electrode of the FWD and an emitter electrode of the IGBT are provided by a common electrode, and a cathode electrode of the FWD and a collector electrode of the IGBT are provided by a common electrode. The RC-IGBT device is included, for example, in an inverter circuit for a pulse width modulation control (PWM control) of a load.
When the RC-IGBT device is included in the inverter circuit, a drive signal to be input to a gate electrode of the IGBT is a signal generally phase-inverted to upper and lower arms. Thus, in a case of inductive load, that is, in a case where a load includes an inductance component, the driving signal is input to the gate electrode of the IGBT even when the FWD is free wheeling. That is, the FWD and the IGBT formed in the one semiconductor substrate are concurrently operated. In order to completely activate the IGBT, the gate electrode of the IGBT is applied with a voltage that is two to three times greater than a threshold voltage and is less than or equal to a rated gate-emitter voltage. For example, the gate electrode is applied with a voltage of 15 V. Thus, in the RC-IGBT device included in the inverter circuit, the FWD is required to be operated in a state where a gate voltage of 15 V is applied, that is, in a state where the IGBT is in operation.
In addition, in the RC-IGBT device, as described in US 2005/0017290 A (for example, FIG. 28) and US 2008/0048295 A (for example, FIG. 16), regions of the IGBT (IGBT regions) and regions of the FWD (FWD region) are alternately arranged in a direction perpendicular to a thickness direction of the semiconductor substrate so that the IGBT and the FWD are operated uniformly and an electric-current concentration can be restricted. When a width of the FWD regions in the one direction is reduced, an electric current distribution during a forward operation is more uniformed and a performance of the FWD can be improved.
The inventors of the present application examined the above-described composition, in which the IGBT regions and the FWD regions are alternately arranged, using a device simulation. As a result of the examination, the inventors found that, when the gate voltage is 15 V, a forward-direction operation of the FWD may be difficult depending on the width of the FWD regions, and a forward voltage Vf may locally increase, that is, a snapback may generate, at a small electric-current region of a forward current “If.” The snapback is remarkable at a low temperature side. When a spike-shaped snapback generates in the forward voltage Vf, the inverter circuit may malfunction. In addition, the forward voltage Vf may increase by the snapback, and a direct-current loss may increase.